Field of the Invention
The invention relates to the forming of containers by blow molding or stretch blow molding of parisons made of plastic material, such as polyethylene terephthalate, with the term “parison” referring to a preform (ordinarily obtained by injection) or an intermediate container that has undergone a preliminary blow-molding operation starting from a preform.
Description of the Related Art
A container comprises a body, generally cylindrical in shape, a shoulder that forms a narrowing from an upper end of the body, an open neck that extends the shoulder for making possible the filling and the emptying of the container, and a bottom that closes the body at a lower end of the former.
The forming is generally carried out in a mold that delimits a cavity bearing the impression of the container. Such a mold commonly comprises a side wall bearing the impression of the body and the shoulder (this side wall being subdivided into two half-molds that are mutually articulated for making it possible to insert a parison into the mold), and a mold bottom bearing the impression of the bottom of the container, positioned in an opening made between the half-molds.
The preform, after having been heated to a temperature that is higher than the glass transition temperature of its material (a preform made of PET, whose glass transition temperature is approximately 80° C., is ordinarily heated to a temperature of higher than 100° C., typically on the order of 120° C.), is introduced hot into the mold. A pressurized fluid (such as air) is then injected therein to flatten the material, made soft by the heating, against the wall and the mold bottom and thus to impart to the preform the impression of the container.
Without heat regulation of the mold at a moderate temperature (on the order of 10° C. to 20° C.), the containers would emerge at a high temperature (higher than the glass transition temperature), would deform and could not be filled immediately, because they would not have sufficient mechanical strength to hold, without deforming, the pressure caused by the filling.
Allowing the containers to cool freely at the exit of the mold cannot be considered for two reasons. First, taking into account current production rates of the machines (on the order of 50,000 containers per hour per machine, representing more than 2,000 containers per hour and per mold), such cooling (that would take approximately one minute) would require the creation of a buffer stock of hundreds of containers, needlessly increasing the size and the complexity of the production line. Next, and primarily, the plastic material left free to cool would undergo an uncontrolled retraction and would thus lose the impression that is given to it by the mold.
This is why most of the molds are provided with a fluid cooling circuit that is designed to keep the wall and the bottom of the mold at a moderate temperature (on the order of 10° C. to 20° C.) in such a way as to set the material while keeping it under pressure to flatten it well against the wall and the bottom of the mold.
The blow molding furthermore requires evacuating the air that is trapped between the preform during forming and the mold. Evacuation is generally provided, on the one hand in the parting line between the two half-molds, and, on the other hand and primarily in the area of the mold bottom, since it is toward it that the air is pushed by the advance of the material front. For this purpose, the mold bottom is ordinarily pierced by one or more pressure-release air vents, more specifically in the zones reached at the end by the material. Thus, the international application WO 00/74925 (Krupp) illustrates a mold bottom that is designed with a petal-shaped bottom: this bottom is equipped with pressure-release air vents formed by perforations made in recessed reserved places of the bottom corresponding to feet of the container.
At the same time that they start to resolve the issue of the evacuation of air, such air vents raise a new issue, linked to their sizing. As a first approximation, it is necessary to maximize their size (i.e., their diameter or their width) since air is to be evacuated as easily as possible.
Then, however, the material will be introduced therein during the blow molding and will form projecting points of uncontrolled size on the surface of the container. As a second approach, it is therefore necessary to reduce the size of the air vents. It is all the more necessary since it was noted that when the air vents are too wide or when the time of cooling under pressure within the mold is brief (which is generally the case), the material is not correctly formed in the area of the air vents, because it undergoes there a retraction during the cooling of the container outside of the mold. Thermographies carried out by the applicant on the containers exiting from the mold actually show hot points located on the zones of the bottom that are located, in the mold, facing the air vents: In these non-thermoregulated zones of the bottom, the material of the container is not cooled.
These hot points are located in the seat of the container (i.e., in the part of the container by which the former is designed to rest on a flat surface). Since any defect of shape of the seat is detrimental to the stability of the container (and therefore to its perceived quality), most of the manufacturers opted for a compromise approach: reducing the size of the air vents to avoid shape defects; increasing the blow-molding pressure to increase the flow rate of air evacuated via the air vents.
Then, however, the problem arises of meeting, without losing production speed, the new requirements of the market as regards the reduction in energy consumption, which call for reducing the blow-molding pressure.